The invention pertains to semiconductor packaging and other device packaging. More particularly, the invention pertains to encapsulation of semiconductors and other devices by stencil printing.
Stencil printing was originally introduced to the semiconductor field for use in placing formations such as solder bumps on the surfaces of semiconductor dies. Essentially, the semiconductor dies are placed under stencils or screens with apertures corresponding to the spots on the surfaces of the die where, for example, solder bumps are to be placed. The depth or height dimension of the stencil is selected to be equal to the desired height of the solder bumps. A viscous solder paste is then applied over the stencil with a wiper or squeegee oriented at an acute angle to the top surface of the stencil. The squeegee horizontally traverses the stencil and pushes the solder paste ahead of it as well as down into the apertures, thus depositing the solder paste in the desired locations of the solder bumps on the surface of the semiconductor die. The stencil is then removed, leaving solder bumps in the desired spots.
Another major use of stencil printing is printing of die attach adhesives. Attempts have been made to use stencil printing in other applications pertaining to semiconductors. Particularly, stencil printing has been attempted for encapsulating semiconductor dies. A stencil printing process for encapsulation of semiconductor dies might involve placing a plurality of dies on a substrate in a regular rectangular pattern such that there are a plurality of parallel vertical streets and a plurality of parallel horizontal streets defining the spaces between the dies. It should be understood that the terms vertical and horizontal are arbitrary and are not intended to define any particular orientation of the streets to the horizon, but merely that the two sets of streets are more or less orthogonal to each other. The terms horizontal and vertical are used herein because they are the terms generally used by persons of skill in the related arts. A stencil is then rested on the substrate so that the dies appear in the aperture or apertures of the stencil.
The width of the streets is selected to provide the desired thickness of encapsulant around the dies. Thus, for example, the width of the streets generally will be selected to be twice the desired encapsulant thickness plus the kerf of the saw blade that will be used to dice the chips. The spacing between the walls of the apertures and the outermost dies that are adjacent the walls need be only as wide as the desired encapsulant thickness since those streets are not shared between two dies, nor do they necessarily have to be sawn for dicing purposes. The stencil apertures have a height equal to the height of the die plus the desired depth of the encapsulant on the top surface of the dies. The area of the apertures is selected to accommodate the desired number of dies.
Each aperture in the stencil typically will contain a plurality of dies laid out in a rectangular pattern and the stencil may have a plurality of such apertures also laid out in a rectangular pattern. However, each die may correspond to a separate aperture in the stencil, if desirable.
A viscous liquid encapsulating material is applied into the apertures as described above using a squeegee that runs over the stencil and forces the material into the apertures in the stencil, covering all sides of the dies therein, except for the side face down and in contact with the substrate. Depending of the particular process, the dies may be placed face up or face down on the substrate. In either event, the surface that is face down on the substrate does not become covered with the encapsulant. However, all of the other sides do. The substrate itself essentially acts as the protective cover for the face down side of the die.
The stencil is then removed and the substrate and plurality of dies are placed in a curing oven to heat cure and harden the encapsulating material. Alternately, UV energy or other methods may be used to cure the encapsulating material. After curing, the substrate is sawn along the horizontal and vertical streets in order to dice the encapsulated semiconductor chips from each other.
In encapsulation applications, the apertures in the stencils will be much larger than in solder bumping applications The size of the apertures when stencil printing is used for encapsulation of semiconductor dies can range as high several inches across each side. Likely aperture sizes include 0.75xe2x80x3xc3x970.75xe2x80x3, 2xe2x80x3xc3x972xe2x80x3 and 2xe2x80x3xc3x976xe2x80x3. On the other hand, the apertures found in stencil printing for solder bumping typically might range from about 50 microns to 100 microns and be circular in shape.
It has been found that encapsulating semiconductor dies by stencil printing tends to leave substantially more and larger voids in the encapsulant than more traditional encapsulation techniques, such as injection molding. Accordingly, stencil printing for semiconductor encapsulation has never been widely commercially accepted.
Several stencil printing machine manufacturers now offer stencil printing machines for solder bumping in which the solder paste is contained in a pressurized vessel in order to push the paste more forcibly into the apertures in the stencil. One such line of machines is the Horizon series of stencil printing machines manufactured by DEK, Inc. of Surrey, England, which includes, among others, the Horizon 265 model. In such stencil printing machines, the solder paste is contained in a closed vessel that can be pressurized to dispense the solder paste. At the bottom of the vessel is a printing head that includes a long, narrow slot with two wipers or squeegees, one on each longitudinal side of the slot. The slot and wipers ride over the stencil forcing the paste out of the slot into the apertures in the stencil. The angle of the slot in the head of the stencil is not adjustable and is set relative to the material handling system that serves stencil/substrate/dies assemblies to the head so that the longitudinal dimension of the slot is 2-3xc2x0 from parallel to one of the two sets of orthogonal streets.
It is an object of the present invention to provide an improved stencil printing machine.
It is another object of the present invention to provide an improved method for encapsulating semiconductor dies or other devices using stencil printing.
It is a further object of the present invention to provide an improved stencil printing apparatus for encapsulating semiconductors.
It is yet a further object of the present invention to provide an improved apparatus for encapsulating semiconductors using stencil printing.
The present invention is a method and apparatus for encapsulating semiconductor dies and other devices using stencil printing techniques. The apparatus includes a pressurized vessel for containing encapsulation material, the apparatus having a stencil head that traverses the stencil, the head including a slot through which the encapsulating material escapes into the apertures of the stencil. The head is angularly adjustable relative to the stencil and thus relative to the streets between semiconductor dies positioned in the apertures of the stencil. Accordingly, the head can be adjusted to the optimal angle for filling both the vertical and horizontal streets between the dies and minimizing the creation of voids in the encapsulant.
The method of the present invention involves encapsulating semiconductor dies using a pressurized stencil printing machine having a slot through which the encapsulating material is forced into the apertures of a stencil and wherein the slot is at a large angle to both the vertical and horizontal streets. Preferably, the angle is greater than 5 degrees. More preferably, the angle is 45 degrees to both the horizontal and vertical streets. Alternately, the angle is 15 degrees to one of the sets of streets (which would, inherently be seventy-five degrees to the other set of streets).